An image forming apparatus such as a copying machine has prevailed in recent years. Among image forming apparatuses, the digital copying machine has achieved a remarkable development. Consequently, sheet postprocessing devices have been developed in earnest for performing postprocessing operations including stapling, punching, and pasting (binding) on sheets ejected out of the copying machine.
The aforementioned sheet postprocessing device is usually provided with a tray for receiving sheets ejected after postprocessing has been performed. In many cases, this tray is arranged so as to ascend and descend in the sheet postprocessing device in response to the fluctuation in the number of ejected sheets. Every time an ascending/descending tray becomes full, the ascending/descending tray descends; therefore, it is possible to eject sheets favorably all the time without causing any defects in the stacking operation. Further, the ascending/descending tray descends so that a larger number of sheets can be stacked than a fixed tray. The following explanation describes a conventional sheet postprocessing device which is provided with the aforementioned ascending/descending tray.
For example, Japanese Laid-Open Patent Publication No.192065/1991 (Tokukaihei 3-192065) discloses a conventional sheet postprocessing device. As shown in FIG. 12, this sheet postprocessing device includes: ejecting rollers 102 and 103 for ejecting a sheet P to an ascending/descending tray 101; the ascending/descending tray 101 which receives the ejected sheet P and is capable of ascending and descending; a tray ascending/descending device 104; and a height detecting means 105.
The tray ascending/descending device 104 moves the ascending/descending tray 101 upward and downward in accordance with a control signal from a CPU (central processing unit, not shown) so as to keep the highest position of the ejected sheets P at a fixed height.
The height detecting means 105 is capable of detecting the highest position of the sheets P stacked on the ascending/descending tray 101 and is provided with detecting levers 106 and 107 which operate independently of each other with different lever lengths. The detecting lever 106, the short lever, is a lever for detecting the height of a small-size sheet such as B5 or A4 sheet. The detecting lever 107, the long lever, is a lever for detecting the height of a large-size sheet such as A3 sheet.
End portions 106a and 107a of the detecting levers 106 and 107 are installed on a pivoting axis 108 so that the levers make a pivoting movement. The pivoting axis 108 is provided in the vicinity of an ejecting outlet 109 of the ejecting rollers 102 and 103 in the same direction of rotating axes of the ejecting rollers 102 and 103. On the other hand, the other end portions 106b and 107b of the detecting levers 106 and 107 are arranged so as to form a contact portion which is always in contact with the upper surface of the sheets P stacked on the ascending/descending tray 101 by the weight of the lever or by pressing force of a spring (not shown). The pivoting axis 108 serves as a supporting point. Here, the detecting levers 106 and 107 are provided in the same direction as the axis with predetermined intervals.
In the vicinity of the pivoting axis 108, photo sensors 110 and 111, which are transmitting type, are provided in parallel with each other with respect to respective positions of levers in the direction of the pivoting axis 108. Further, light-shielding portions 112 and 113 are integrally fixed on the end portions 106a and 107a of the detecting levers 106 and 107 respectively. When the detecting levers 106 and 107 rotate by a predetermined angle while the sheet P is being ejected, the light-shielding portions 112 and 113 shield light on detecting sections 110a and 111a of the photo sensors 110 and 111. Therefore, the photo sensors 110 and 111 detect the light-shielding portions 112 and 113, which make a pivoting movement with the detecting levers 106 and 107, by the use of the detecting sections 110a and 111a so that it is possible to detect the position of the highest surface of the sheets P stacked on the ascending/descending tray 101. Successively, the photo sensors 110 and 111 send a detected result to the CPU.
With the aforementioned arrangement, a sheet P, which is subjected to postprocessing operations, including stapling and others in the sheet postprocessing device, is ejected onto the ascending/descending tray 101 through the ejecting rollers 102 and 103. Here, in the case when the sheet P is a small-size sheet, the leading portion of the sheet P slips between the ascending/descending tray 101 and the detecting lever 106 so as to move the end portion 106b of the detecting lever 106 upward. On the other hand, in the case when the sheet P is a large-size sheet, the leading portion of the sheet P initially slips between the ascending/descending tray 101 and the detecting lever 106 so as to move the end portion 106b of the detecting lever 106 upward. Then, the leading portion of the sheet P slips between the ascending/descending tray 101 and the detecting lever 107 so as to move the end portion 107b of the detecting lever 107 upward. Consequently, the detecting levers 106 and 107 respectively make a pivoting movement around the pivoting axis 108 in the direction of C (clockwise) as shown in FIG. 12. Additionally, the following description explains a case wherein the sheet P is a small-size sheet. Even in the case when the sheet size is large, the principle is the same as that of a small-size sheet.
As the ejecting operations are performed successively, the thickness of the sheets P gradually increases. When the detecting lever 106 makes a pivoting movement by a predetermined angle and the light-shielding portion 112 shields light on the detecting section 110a of the photo sensor 110, the photo sensor 110 recognizes that the upper surface of the sheets P stacked on the ascending/descending tray 101 has reached a predetermined height, and the photo sensor 101 sends a detection signal to the CPU. The CPU, which receives the detection signal, sends a signal to the tray ascending/descending device 104 so as to move the ascending/descending tray 101 downward by a predetermined amount, and then, the next sheet P can be ejected onto the ascending/descending tray 101.
Furthermore, when the ascending/descending tray 101 descends, the detecting lever 106 makes a pivoting movement in the direction of D (counterclockwise) as shown in FIG. 12 while being in contact with the upper surface of the sheets P by the weight of the lever or by pressing force of a spring (not shown). The pivoting axis 108 serves as a supporting point. At this time, since the light-shielding portion 112 is fixed on the detecting lever 106, the light-shielding portion 112 pivots in the same direction so as to remove the shield provided by the light-shielding portion 112 from the detecting section 110a. This operation allows the photo sensor 110 to detect the position of the upper surface of a succeeding sheet. The same operation is repeated in the following process. When the ascending/descending tray 101 reaches the lowest position, the tray of the sheets P has become full.
However, in the case when a very soft sheet (very thin sheet) is used, the aforementioned conventional arrangement causes inconveniences as follows:
Namely, as shown in FIG. 13, after having being ejected from the ejecting rollers 102 and 103, the leading portion of the very soft sheet cannot slip between the ascending/descending tray 101 and the detecting lever 106; therefore, it is not possible to move the detecting lever 106 upward. This is because the weight of the detecting lever 106 is too heavy load for the soft sheet P. As a result, as shown in FIG. 13, the sheet P is warped, thereby causing defects in stacking.
Moreover, in addition to the height detecting means 105 serving as a mechanical detecting means, an optical detecting means is also available for detecting the position of the upper surface of the sheets P. The optical detecting means, for example, receives reflected light by projecting light on the upper surface of the sheets P so as to detect the upper surface position of the sheets P in accordance with intensity of the received light. However, in order to detect the position correctly, it is further favorable to use the mechanical detecting means rather than the optical means. The reasons are as follows:
(1) Since the optical detecting means performs a detecting operation in accordance with light reflected from the surface of the sheet P which is stacked on the ascending/descending tray, the detecting operation is performed stably as long as the surface of the sheet P which reflects the light is always uniform. However, each sheet P on the ascending/descending tray differs in a degree of curling caused by passing through the fixing device and in hardness; therefore, the intensity of reflected light and the direction of reflection are not uniform in a practical operation. As a result, a detection using the optical detecting means is not reliable enough at this time. PA1 (2) For example, in the case when light is projected on an image surface of the sheet P, which is black all over, most light is absorbed thereon. Consequently, it is not possible to obtain a correct quantity of reflected light. PA1 (3) For example, in the case when the sheet P is a transparent sheet used for OHP (overhead projector) and others, projected light passes through the transparent sheet. Consequently, it is not possible to obtain reflected light. PA1 a sheet detecting section for detecting the existence of a sheet to be ejected onto an ascending/descending tray, PA1 a contact member which is separably contact with the upper surface of ejected sheets on the ascending/descending tray, and PA1 an upper-surface regulating section which moves the contact member so as to allow the contact member to be in contact with the upper surface of the ejected sheets in order to regulate the height of the upper surface of the ejected sheets in the case when no sheet to be ejected exists and which also moves the contact member so as to allow the contact member to separate from the upper surface of the ejected sheets in the case when a sheet to be ejected exists.